Semiconductor technology has recently included frequency and phase control circuits which depend on the ratio of pairs of capacitors and which provide superior accuracy compared to earlier technology. One of the main limitations of this new technology is the range of capacitor ratios available on the semiconductor chip for a given area. This invention relates to a capacitor ratio multiplier which increases the range of accurate capacitor ratios available for a given chip area thus increasing the range of frequency and gain of filters and phase controls for a given cost.
A common application for a capacitor ratio multiplication is in a filter circuit formed on a monolithic microelectronic chip. Prior attempts to form capacitors having very small capacitance values on monolithic micro-electronic chips have been fairly unsuccessful due to the inability to accurately provide the small chip areas required to produce such small capacitance values. Contemporary chip manufacturing technology has not been able to produce and accurately measure such small chip areas. Additionally, parasitic capacitances in the vicinity of the small capacitors influence the capacity values of such capacitors beyond the amount tolerable in a designed circuit.
There are several circuit techniques available that can be utilized for reducing the effective capacitance value of a capacitor. For example, a number of series-connected capacitors may be used in replacement of a single small value capacitor. This technique, however, requires the use of larger chip areas due to the areas required for each individual series-connected capacitor.
Circuits that include switched capacitors have transfer functions that depend on the switched capacitor capacitance value and the capacitor switching frequency. The range of capacitance values available in such circuits therefore depends heavily on the permissible variation in the switching frequency. Such circuits have a narrow range of equivalent capacitance values that the switched capacitors can provide. Attempts to increase and provide additional range of parameter values than practically available have fallen short of their design goals.
One application of switched capacitors is in the simulation of "resistors" in conjunction with other capacitors and amplifiers for providing active RC filters. The design of these filters is considerably dependent upon the range of "resistance" values obtainable by switched capacitor techniques. Attempts to increase the range of equivalent resistance values that a switched capacitor can simulate have not, however, proven very successful.